Subsurface rotary expansible drilling tools



y 1957 A. w. KAMMERER 2,

SUBSURFACE ROTARY EXPANSIBLE DRILLING TOOLS Filed Nov. 7, 1955 2 Sheets-Sheet 2 I10. 6. L i i i J 72 r 7a L INVENTOR. I flea/me "(Emma/e55 United SUBSURFACE ROTARY EXPANSIBLE DRILLING "rooLs The present invention relates to rotary drilling tools of the expansible type, particularly useful in severing and milling away casing sections located in well bores, and also for enlarging the diameter of such well bores.

Hydraulically actuated expansible types of rotary drilling tools are employed in enlarging well bores and in severing and milling away casing sections disposed in well bores. The cutter elements of the particular'tool being used are in retracted position until the tool is lowered on a tubular drilling string in the-well bore to the point at which the tool is to become operative. When such point is reached, fluid under pressure is pumped through the drill string and the tool to hydraulically expand the cutters to the proper extent.

In effecting cutter expansion, it is essential to rotate the tool while expansion is occurring, to drill away the formation laterally and enlarge its diameter, which enables the cutters to move outwardly to their maximum extent. When employed to cut away casing, the tool is rotated while the cutters are being expanded. Expansion of the cutters can only proceed as a result of milling away and eventually severing the casing, followed by removal of any cement or formation material behind the casing. Expansion of the cutters produces a transverse shoulder on which the cutters can then rest, the shoulder being provided either in the formation itself, or by the severed ends of the casing therebelow.

Once the transverse shoulder has been formed, the necessary drilling weight can be imposed on the cutters by means of the drill string. However, during the initial production of the shoulder or severing of the casing, the only force available is that due to the pressure of the drilling fluid being pumped through the drill string and tool. It is desirable to pump drilling fluid through the tool during the expansion of the cutters to remove the cuttings and cool the cutters, but such pumping of fluid is accompanied by a reduction in the fluid pressure available for cutter expansion. Maximum hydraulic force could be secured by preventing any fluid from escaping from the tool. This is undesirable since removal of cuttings and cooling of the cutters would not be accomplished, impeding operation of the tool and shortening its life.

Accordingly, it is an object of the present invention to provide a hydraulically expansible rotary drilling bit in which a substantially greater hydraulic force is available for expanding the cutters outwardly, while drilling fluid is being pumped through the tool for the purpose of cooling the cutters and flushing the cuttings to the top of the well bore.

Another object of the invention is to provide a hydraulically expansible rotary drill bit, in which fluid being pumped through the bit is throttled during the period of cutter expansion, to provide circulation around the cutters and a greater fluid pressure in the tool to insure expansion of the cutters against relatively high resistance, the throttling action being substantially removed after the cutters have been expanded to permit greater Patent ice flow of circulating fluid around the cutters and in the well bore. If deemed necessary, the throttling action can be reestablished readily, and subsequently removed without removing the bit from its location in the well bore.

This invention possesses many other advantages, and has other objects which may be made more clearly apparent from a consideration of a form in which it may be embodied. This form is shown in the drawings accompanying and forming part of the present specification. It will now be described in detail, for the purpose of illustrating the general principles of the invention; but it is to be understood that such detailed description is not to be taken in a limiting sense, since the scope of the inven tion is best defined by the appended claims.

Referring to the drawings:

Figure 1 is a longitudinal section through a rotary well drilling bit exposed in a well casing, with its cutter members in retracted position;

Figure 2 is a view similar to Figure l disclosing the cutters in their maximum expanded position;

Figure 3 is an enlarged fragmentary longitudinal section through a portion of the apparatus, with a cutter member in expanded position;

Figure 4 is an enlarged cross-section taken along the line 4--4 on Figure 3;

Figure 5 is an enlarged cross-section taken along the line 55 on Figure 3;

Figure 6 is an enlarged longitudinal section through a portion of the apparatus after fluid has been pumped therethrough for a substantial period.

As shown in the drawings, a rotary expansible drill bit A is secured to the lower end of a string of drill pipe B that extends to the top of a well bore C, and by means of which the drill bit is lowered within a string of well casing D to a region therein at which a desired operation is to commence. By way of example, the tool A will be described in connection with a casing severing and milling operation. It is to be understood, however, that the tool A has other uses, such as enlarging the diameter of a well bore below well casing.

The upper portion of the rotary drill bit consists of a mandrel 10 having an upper pin 11 threadedly connected to the lower end of the string of drill pipe B. This mandrel includes an upper kelly or drill stem member 12 slidably splined to the main body 13 of the drill bit. The exterior of the lower portion 14 of the kelly is non-circular in shape, being telescopically received in a companion non-circular socket 15 formed in the main bit body 13.

Specifically, the kelly exterior and the socket 15 may be of hexagonal shape, to enable the kelly 12 to be moved longitudinally With respect to the body 13, while still being capable of transmitting rotary motion to the' body.

The mandrel It has a limited range of longitudinal movement within the body, its downward movement being determined by engagement of the lower end of the kelly with an inwardly directed body shoulder 17 and its upward movement being limited by engagement of an-external shoulder or piston portion 13 of the kelly with a cylinder head 19 secured to the body. The upper end of the head has a flange 20 engaging a body shoulder 21, the flange being prevented from moving upwardly of the body by split snap retainer rings 22 fitting in a body groove 23 and overlying the flange 20. An annular guide 24 is releasably secured to the body 13 by a split snap ring 25 above the retainer rings.

The body 13 has a plurality of expansible parts mounted on it. These include cutter supporting members 26 pivotally mounted in body slots 27 on hinge pins 28 suitably secured to the body, to prevent their loss therefrom.

Each cutter supporting member 26 depends from the t ghinge pin 28 and carries a drag or milling cutter structure 29 at its lower end, which structure can assume any desirable form.

The cutter supporting members 26 and the cutter structures 29 tend to occupy a retracted position substantially entirely within the confines of the main body 13 of the bit. These cutter supporting members and the cutter structures are expandible outwardly, to sever the casing D and operate upon the upper end E of the severed casing therebelow. To accomplish the expansion, each cutter supporting member 26 has an inclined expander surface 30 on its inner portion below the hinge pin 28 which tapers in a downward and inward direction. Each expander surface terminates in a lock surface 31 formed on a lock portion 32 of the cutter supporting member. The outward expansion is accomplished by producing relative longitudinal movement between the mandrel 1G and the bit body 13, which will produce relative longitudinal movement between the cutter supporting members 26 and the tubular member 33 of the mandrel 10. This tubular member includes a lower portion 34 slidable within a guide bushing 35 mounted in a bridge 36 secured to the body and extending across the body slots 27. This guide bushing 35 i disposed below the lock portions 32 of the cutter supporting members 26.

Located initially substantially above the guide bushing 35 and below the hinge pins 28, and in cutter member recesses 37, is a mandrel lock and expander 38 which has outer surfaces 39 adapted to engage the expander surfaces 30 and the lock surfaces 31. The lock and expander 38 may be formed integral with the tubular member 33, the upper end of the latter being pivoted within a socket 40 formed in the lower portion 14 of the kelly 12. An enlarged boss 41 on the tubular member 33 engages a downwardly facing shoulder 42 of the kelly, the tubular member being held against this shoulder by a suitable split retainer or lock ring 43 snapped into an internal groove 44 encompassing the kelly socket and engaging the lower end of the tubular member boss 41..

Drilling mud or other fluid can pass down through the central passage 45 in the kelly or drill stem 12 and into the central passage 46 extending completely through the tubular member 33. Leakage of fluid around the exterior of the tubular member 33 is prevented by a suitable side seal ring 47, such as a rubber O ring, in a peripheral groove 48 in the kelly, which engages the exterior wall of the boss 41.

Assuming that the body 13 of the tool is elevated relatively along the tubular mandrel 10 the inclined expander surfaces 30 of the cutter supporting members 26 will be shifted upwardly along the lock and expander portion 38 of the tubular member 33. During such upward shifting, the cutter supporting members 26 and the cutter structures 29 carried thereby will be pivoted about the hinge pins 28 and urged in an outward direction. The upward movement of the body 13 with respect to the tubular mandrel 10 can continue until the cutter structures 29 have been shifted outwardly to their fullest extent, as determined by engagement of stop shoulders 49 on the cutter supporting members 26 with companion shoulders 54) formed in the body on opposite sides of the body slots 27. When such engagement occurs, the lower end 16 of the kelly portion 12 of the tubular mandrel will engage the body shoulder 17, and the lock and expander 38 on the tubular member 33 will be disposed behind and in engagement with the lock portions 32 on the cutter supporting member 26.

It is to be noted that the surfaces 31 of the lock portions 32 of the cutter supporting members 26 and the companion surfaces 39 on the lock and expander portion 38 of the tubular member are substantially parallel to the axis of the drill bit, to prevent the reactive forces of the casing or formation on the cutter structures 29 from moving the latter inwardly. Asa practical matter, it is preferred that the coengaging lock surfaces 31, 39

' 45 through the kelly and the cylinder space.

be inclined slightly in a downward direction toward the axis of the tool, to insure release of the lock and the expander portion 38 from the cutter supporting members 26 when the latter and the cutter structures 29 are to be shifted to retracted position.

The relative longitudinal movement between the tubular mandrel 10 and the body 13 of the tool is accomplished hydraulically. Thus, the piston orenlarged portion 18 on the drill stem 12 is received within a counterbore 51 formed on the upper portion of the body of the tool. This upper portion actually constitutes a cylinder 52 having a cylindrical wall 53 extending from a lower shoulder 54, defining the bottom of the counterbore 51, to the cylinder head 19.

A confined cylinder space 55 is formed between the piston portion 18 of the kelly, the periphery of the kelly above the piston, and the cylinder 52. A suitable packing or side seal ring 56 may be disposed in a suitable piston ring groove 57 formed in the piston 18, which is adapted to slidably seal against the cylindrical wall 53 of the cylinder 52. Fluid is thereby prevented from passing in a downward direction between the pistonand the cylinder. Similarly, fluid is prevented from passing in an upward direction out of the annular cylinder space 55 by an inner side seal ring 58 carried by an internal groove 59 in the cylinder head 19 and slidably and sealingly engaging the periphery of the kelly 12 above the piston 18, and also by an outer side seal ring 60 disposed in an external groove 61 in the head 19 and sealingly engaging the cylinder wall 53.

Fluid under pressure in the string of drill pipe B and in the tubular mandrel passage 45 can be fed into the cylinder space 55 through one or more side ports 62 establishing communication between the central passage Such fluid under pressure is developed, in the form of invention disclosed in the drawings, by virtue of the fact that the passage 46 through the tubular member 33 of the mandrel 10 is of a restricted diameter as compared to the passage 45 through the kelly portion of the mandrel. As a result, the pumping of drilling mud, or other fluid, at an adequate rate through the apparatus will build up a back pressure of fluid in the passage 45, which pressure will be imposed on the fluid in the cylinder space 55, acting upon the cylinder head 19 to urge the body 13 of the tool in an upward direction with respect to the tubular mandrel 10, to secure the outward expansion of the cutter supporting members 26 and cutter structures 29 to their fullest extent, as above described.

In addition to effecting outward expansion of the cutter supporting members 26 and cutter structures 29 to their fullest extent, the fluid will pass downwardly out of the passage 46 through the tubular member 33 and will then flow upwardly around the cutters 29 to remove the cuttings and cool the cutters themselves, The cuttings will be flushed upwardly through the annulus between the drill pipe B and the casing D to the top of the hole for accurate disposal. It is desirable to have such circulating fluid available for cooling the cutters and for removing the cuttings, as expansion of the cutters occurs. However, it is also desirable to insure the presence of a proper pressure in the cylinder space 55 so that a large total force is available for forcing the cutters 29 outwardly, particularly when such cutters are used to sever Well casing D.

To insure the presence of adequate pressure in the cylinder space 55, and the circulation of fluid during the easing severing operation, or, in general, the outward expansion of the cutters, the flow of fluid through the passages 45 and 46 is further restricted by providing a throttle device in the tubular member 33 itself. As specifically disclosed, the lower portion of the tubular member 33 has a counterbore 76 to receive a tubular sleeve 71, preferably made of a hard material, such as tungsten carbide, in order to resist erosive wear. This 71 and completely out of the latter.

'5 sleeve is suitably secured in the lower end of the tubular member 33 as by welding, its upper portion 72 having an inside diameter substantially equal to the inside diameter of the tubular member passage 46. The lower portion 73 of the sleeve extends inwardly from opposite sides to provide a non-circular fluid passage portion 74 of restricted area, which area is further restricted by the engagement of an erodible valve element 75 with the upwardly facing seat 76 that the inwardly directed portion 73 of the sleeve provides. It is to be noted that the erodible valve element 75 which may be in the form of a ball made of mild steel, or other suitable material that is subject to a relatively slow erosion under the influence of fluid acting thereon, engages the seat 76 at diametrically opposite points (Figs. 4 and 5), and that there is a space or an orifice passage 77 around the ball 75 and betweenthe ball and the sleeve 71 of relatively restricted area through which fluid can pass. Thus, the ball 75 will be urged in a downward direction against its companion seat 76 while fluid is being pumped through the drill pipe B and the mandrel passages 45, 46, the fluid flowing through the relatively small choke orifice space or spaces 77 around the ball, before discharging from the tungsten carbide sleeve 71. Since the effective area of the orifice 77 is relatively small, a much greater back pressure will be built up in the mandrel passages 45, 46, ports 62 and cylinder space 55 than if the ball 75 were absent. Accordingly, a much greater hydraulic force is available for lifting the body '13 and the cutters 29 with respect to the mandrel 10, to cause outward expansion of the cutters 29. The cutters 29 can be urged against the wall of the well casing D and into the casing material itself wtih a much greater force than in the absence of the erodible ball valve element 75. Despite the presenceof the erodible ball valve element, 'asuflicient quantity of fluid can still flow therearound for upward movement in the well bore around the cutters 29 for the purpose of removing the cuttings and flushing them to the top of the hole.

After the cutters 29 have been expanded outwardly to their fullest extent, a shoulder E is provided bythe severed ends of the well casing D, and by the formation itself on which the cutters 29 can rest (Fig. 2). At this time fluidunder pressure within the cylinder space 55 is no longer necessary for application on the cutters, since drilling weight imposed on the drill pipe B, mandrel and body 13 of the tool will provide the necessary force of the cutters against the severed end E of the casing and the formation shoulder, to mill away the casing in a downward direction as rotation of the tool Aproceeds. At this time, however, it is desirable to have a greater quantity of drilling fluid pumped down through the drill pipe B and out of the tool for upward passage around the cutters and the drilling region, and through the annulus around the drill pipe to the top of the hole.

The ball valve element, being erodible, willgradually be worn away as fluid is pumped therearound. Under conditions usually encountered in the well bore, substantial erosion of the valve element 75 will take place by the time the cutters 29 have been hydraulically expanded outwardly to their maximum position. As such erosion occurs, theorifice area between the valve element 75 and the sleeve 71 increases, permitting greater quantities of fluid to be pumped from the mandrel 10 into the well bore. Eventually, erosion of the valve element 75 will occur to such an extent that it can be forced completely through the lower fluid passage portion 74 of the sleeve When this occurs, the full area of the lower fluid passage portion 74 is available through which fluid can flow.

In the event that the erodible valve element 75 has been eroded and pumped out of the apparatus before the cutters 29 have severed the casing, which will reduce the fluid pressurein-the cylinder space 55, and should it be necessary to reestablish the higher fluid pressure,

allthat need be done is to pump another erodible valve element down the drill pipe B and into engagement with the sleeve seat 76, which will again throttle the flow of fluidthrough the mandrel 10 to a considerable extent. This second valve element will then be subject to erosion as fluid is pumped through the apparatus, eventually being forced through the lower fluid passage 74 and out of the apparatus. By this time, the cutters 29 will have been expanded outwardly to their fullest extent, making it unnecessary for the greater fluid pressure to be provided in the cylinder space.

In the operation of the apparatus A, it is run in the well casing D with the cutter supporting members 26 and the cutter structures 29 in their initial retracted positions, such as disclosed in Fig. 1. At this time, if desired, the erodible valve element 75 may be disposed in the tubular member passage 46. Despite its presence in the passage 46, the ball valve element 75 will be raised from its seat 76 by the fluid in the Well bore, to permit the latter to flow upwardly through the mandrel passage 45, 46 and into the drill pipe B enabling the drill pipe to automatically fill with the well bore fluid. When the apparatus has been lowered to the location in the well casing at which the severing and milling operation is to take place, the pumps at the top of the well bore are started, the fluid shifting the erodible valve element 75 down against its seat 76 and throttling the flow of fluid through the mandrel passage to a substantial extent. A back pressure is built up in the passages 45, 46, as well as in the fluid in the ports 62 and the cylinder space '55, which pressure will act upon the cylinder head 19 to urge the body 13, cutter supporting members 26 and cutter structures 29 in an upward direction with respect to the mandrel 10. 7

During such upward movement, the expander surfaces 30 of the cutter supporting members 26 are brought to bear against the lock and expander portion 38 of the mandrel 10, the cutter structures 29 being urged in an outward direction against the wall of the well casing D. The drill pipe B and the rotary drill bit A are rotated at the proper speed while fluid is being pumped through.

the apparatus, the cutter structures 29 beginning to mill or cut away the casing and being shifted outwardly as the cutting operation proceeds. As severing of the casing continues, the hydraulic force acting upon the body 13 raises it and the cutter supporting members 26 and'the cutter structures 29 to a further extent, until the cutters 29 are fully expanded outwardly to the maximum extent, asv determined by engagement of the supporting member stop shoulders 49 with the companion stop shoulders 50, and the lower end 16 of the kelly portion 12 of the mandrel with the body shoulder 17. With the parts in this position, the lock portions 32 of the cutter supporting members 26 will bear against the lock and expander portion 38 of the tubular member 33, to preclude inadvertent partial retraction of the cutter structures 29 from their fully expanded position. I

A substantial period which, for example, may be about one or two hours, may be required before the cutters 29 will sever the casing D completely, and be shifted outwardly to their fully expanded position. During all of this time, fluid is being pumped down through the drill pipe B and the mandrel passages 45, 46, acting upon the erodible valve element 75 and gradually eroding it or reducing its effective diameter. During all of this time, the flow through the apparatus has been substantially throttled, to insure the existence of proper back pressure in the cylinder space 55 for hydraulically expanding the cutters 29 outwardly. By the time the cutters have been expanded outwardly to their fullest extent, the valve element 75 will have been eroded sufficiently to allow a 'still greater amount of fluid to be discharged fromthe mandrel 10 to remove the cuttings and cool the cutters 29. As a matter of fact, the erodible valve element 75 may have been reduced in size to such an extent as to be ejected completely from the mandrel passage 46, 74. However, such erosive action does not occur on the sleeve 71 since it is made of much harder material than the throttle valve element 75 itself.

By the time substantial erosion has occurred on the valve element 75, or it has been completely ejected from the apparatus, the cutters 29 have been expanded outwardly to their fullest extent. Downweight of the proper amount can now be imposed on the string of drill pipe B, this downweight being transmitted through the kelly 12 to the body shoulder 17, and from the body through the stop shoulders 59, 49 directly to the supporting members 26 and the cutter structures 29, urging such structures against the severed upper end B of the casing therebelow, as well as against the formation material itself, which might be dispersed around and against the casing. With the downweight applied, the drill pipe B and apparatus A are rotated, to drill away the casing D in a downward direction, as well as to enlarge the well bore C. During the drilling action, the circulating fluid is pumped in a comparatively unimpeded manner through the drill pipe 5 and mandrel passages 45, 46, to act upon the cutting region for the purpose of removing the cuttings, as well as cool the cutter elements 29 themselves.

In the event it is desired to retrieve the apparatus from the well bore after a sufficient length of easing has been milled away, or as a result of the cutter structures 29 becoming worn, it is only necessary to discontinue the pumping of the drilling fluid through the drill pipe B and the apparatus A, to relieve the pressure in the cylinder 52, and elevate the drill pipe. Such elevating movement will elevate the tubular mandrel 10 with respect to the body 13 and the cutter supporting members 26, to raise the lock and expander portion 38 above the expander surfaces 39, whereupon the cutter structures 2& can drop back to their retracted position, such as disclosed in Fig. 1. In the event the cutters 29 are reluctant to move to such position, the outer surfaces of the cutter supporting members 26 will engage the lower end P of the severed casing thereabove, upon elevation of the apparatus in the casing, which will force the cutter supporting members 26 inwardly, enabling the apparatus to be elevated through the casing to the top of the well bore.

It is, accordingly, apparent that an apparatus has been provided in which the cutters are expanded outwardly hydraulically, with a relatively large pressure available for effecting such expansion. At the same time, circulating fluid is always present in adequate amounts to insure the removal of the cuttings from the well bore and the cooling of the cutter members. When the need for hydraulic pressure no longer exists, the fluid passage through the apparatus has a much greater effective area, allowing relatively large volumes of fluid to be pumped therethrough, for the purpose of removing the cuttings and cooling the cutters.

The inventor claims:

I. In a rotary drill bit to be lowered in a well bore: a main body having a fluid passage through which drilling fluid can flow; cutter means mounted on said body for expansion laterally outward of said body; hydraulically actuated means responsive to the pressure of fluid in said passage for expanding said cutter means laterally outward of said body; readily erodible means in said passage providing a choke orifice effecting a back pressure in said passage for action on said hydraulically actuated means while said hydraulically actuated means is expanding said cutter means. said erodible means wearing away as fluid is pumped through said passage to substantially enlarge said choke orifice while said hydraulically actuated means is expanding said cutter means.

2. In a rotary drill bit to be lowered in a well bore: a main body having a fluid passage through which drilling fluid can flow; cutter means mounted on said body for expansion laterally outward of said body; hydraulically actuated means responsive to the pressure of fluid in said passage for expanding said cutter means laterally outward of said body; an erodible member in said passage to choke the flow of fluid therein while fluid is being pum ed through said passage and past said erodible member and cause a back pressure to be built up in said passage for action on said hydraulically actuated means, said erodible member wearing away as fluid is pumped through said passage for eventual removal from said passage to fully open the same.

3. In a rotary drill bit to be lowered in a well bore: a main body having a fluid passage through which drilling fluid can flow; cutter means mounted on said body for expansion laterally outward of said body; hydraulically actuated means responsive to the pressure of fluid in said passage for expanding said cutter means laterally outward of said body; said passage having a seat therein; an erodible member in said passage movable downwardly into engagement with said seat to provide an orifice to choke the flow of fluid in said passage and cause a back pressure to be built up in said passage for action on said hydraulically actuated means, said erodible member wearing away as fluid is pumped through said passage to substantially enlarge said orifice.

4. In a rotary drill bit to be lowered in a well bore: a main body having a fluid passage through which drilling fluid can flow; cutter means mounted on said body for expansion laterally outward of said body; hydraulically actuated means responsive to the pressure of fluid in said passage for expanding said cutter means laterally outward of said body; said passage having a seat therein; an erodible ball in said passage movable downwardly into engagement with said seat to provide an orifice to choke the flow of fluid in said passage and cause a back pressure to be built up in said passage for action on said hydraulically actuated means, said ball wearing away as fluid is pumped through said passage to substantially enlarge said orifice.

5. In a rotary drill bit to be lowered in a well bore: a main body having a fluid pasage through which drilling fluid can flow; cutter means mounted on said body for expansion laterally outward of said body; hydraulically actuated means responsive to the pressure of fluid in said passage for expanding said cutter means laterally outward of said body; said passage having a sleeve therein made of material having high erosion resistance, said sleeve providing a seat; a readily erodible member in said passage movable downwardly into engagement with said seat to provide an orifice to choke the flow of fluid in said passage and cause a back pressure to be built up in said passage for action on said hydraulically actuated means, said erodible member wearing away as fluid is pumped through said passage to substantially enlarge said orifice.

6. In a rotary drill bit to be lowered in a well bore: a main body having a fluid passage through which drilling fluid can flow; cutter means mounted on said body for expansion laterally outward of said body; hydraulically actuated means responsive to the pressure of fluid in said passage for expanding said cutter means laterally outward of. said body; said passage having a sleeve therein made of material having high erosion resistance, said sleeve providing a seat; a readily erodible steel ball in said passage movable downwardly into engagement with said seat to provide an orifice to choke the flow of fluid in said passage and cause a back pressure to be built up in said passage for action on said hydraulically actuated means, said ball wearing away as fluid is pumped through said passage to substantially enlarge said orifice.

7. In a rotary well drilling bit to be lowered in a well bore: a main body; cutter means mounted on said body for expansion laterally of said body; a mandrel connecti- .ble to a drill string and slidably splined to said body;

coengageable expander means on said mandrel and cutter means; said mandrel having a fluid passage to receive fluid from the drill string; hydraulically actuated means on said body responsive to the pressure of fluid in said passage for elevating said body and cutter means with respect to said mandrel to cause said expander means to expand said cutter means laterally outward; and erodible means in said passage providing a choke orifice effecting a back pressure in said passage for action on said hydraulically actuated means, said erodible means Wearing away as fluid is pumped through said passage .to substantially enlarge said choke orifice.

8. In a rotary well drilling bit to be lowered in a well bore: a main body; cutter means mounted on said body for expansion laterally of said body; a mandrel connectible to a drill string and slidably splined to said body; coengageable expander means on said mandrel and cutter means; said mandrel having a fluid passage to receive fluid from the drill string; hydraulically actuated means on said body responsive to the pressure of fluid in said passage for elevating said body and cutter means with respect to said mandrel to cause said expander means to expand said cutter means laterally outward; an erodible member in said passage to choke the flow of fluid therein and cause a back pressure to be built up in said passage for action on said hydraulically actuated means, said erodible member wearing away as fluid is pumped through said passage for eventual removal from said passage to fully open the same.

9. In a rotary well drilling bit to be lowered in a well bore: a main body; cutter means mounted on said body for expansion laterally of said body; a mandrel connectible to a drill string and slidably splined to said body; coengageable expander means on said mandrel and cutter means, said mandrel having a fluid passage to receive fluid from the drill string; hydraulically actuated means on said body responsive to the pressure of fluid in said passage for elevating said body and cutter means with respect to said mandrel to cause said expander means to expand said cutter means laterally outward; said passage having a seat therein; an erodible member in said passage movable downwardly into engagement with said seat to provide an orifice to choke the flow of fluid in said passage and cause a back pressure to be built up in said passage for action on said hydraulically actuated means, said erodible member wearing away as fluid is pumped through said pasage to substantially enlarge said orifice.

10. In a rotary well drilling bit to be lowered in a Well bore: a main body; cutter means mounted on said body for expansion laterally of said body; a mandrel connectible to a drill string and slidably splined to said body; coengageable expander means on said mandrel and cutter means; said mandrel having a fluid passage to receive fluid from the drill string; hydraulically actuated means on said body responsive to the pressure of fluid in said passage for elevating said body and cutter means with respect to said mandrel to cause said expander means to expand said cutter means laterally outward; said passage having a seat therein; an erodible ball in said passage movable downwardly into engagement with said seat to provide an orifice to choke the flow of fluid in said passage and cause a back pressure to be built up in said passage for action on said hydraulically actuated means, said ball wearing away as fluid is pumped through said passage to substantially enlarge said orifice.

11. In a rotary Well drilling bit to be lowered in a well bore: a main body; cutter means mounted on said body for expansion laterally of said body; a mandrel connectible to a drill string and slidably splined to said body; coengageable expander means on said mandrel and cutter means; said mandrel having a fluid passage to receive fluid from the drill string; hydraulically actuated means on said body responsive to the pressure of fluid in said passage for elevating said body and cutter means with respect to said mandrel to cause said expander means to expand said cutter means laterally outward; said passage having a sleeve therein made of material having high erosion resistance, said sleeve providing a seat; a readily erodible member in said passage movable downwardly into engagement with said seat to provide an orifice to choke the flow of fluid in said passage and cause a back pressure to be built up in said passage for action on said hydraulically actuated means, said readily erodible member wearing away as fluid is pumped through said passage to substantially enlarge said orifice.

12. In a rotary well drilling bit to be lowered in a well bore: a main body; a cutter means mounted on said body for expansion laterally of said body; a mandrel connectible to a drill string and slidably splined to said body; coengageable expander means on said mandrel and cutter means; said mandrel having a fluid passage to receive fluid from the drill string; hydraulically actuated means on said body responsive to the pressure of fluid in said passage for elevating said body and cutter means with respect to said mandrel to cause said expander means to expand said cutter means laterally outward; said passage having a sleeve therein made of material having high erosion resistance, said sleeve providing a seat; a readily erodible steel ball in said passage movable downwardly into engagement with said seat to provide an orifice to choke the flow of fluid in said passage and cause a back pressure to be built up in said passage for action on said hydraulically actuated means, said ball wearing away as fluid is pumped through said passage to substantially enlarge said orifice.

References Cited in the file of this patent UNITED STATES PATENTS 2,049,450 Johnson Aug. 4, 1936 2,284,170 Santiago May 26, 1942 2,330,564 Dyer Sept. 28, 1943 2,347,204 Lindsey Apr. 25, 1944 2,457,628 Baker Dec. 28, 1948 

